In the drying of pourable goods, especially in the drying of wood chips and strands, directly heated rotary drums are used. The bulk material and the drying gas are introduced at the inlet of the rotary drum and the dried bulk materials and the drying gas are withdrawn at the outlet.
To generate a good heat transfer from the drying gas to the bulk material, it is known from the book: K. Kroll "Trockner und Trocknungsverfahren", Springer-Verlag 1959, pages 475 to 479, to provide the rotary drums with baffles. The baffles serve to subdivide the bulk material into a number of masses to distribute the bulk material uniformly over the drum cross section, to provide a greater surface, and to allow the gas stream to pass through the bulk material repeatedly in a transverse manner. The there-described cross baffles have been used by the applicant frequently for the drying of wood chips.
Cross baffles are also known from DE-B 23 62 725 and DE-B 29 11 137. They are not well suited for the drying of strands, wood pieces of widths of 5 to 50 mm and lengths of 75 to 150 mm, from which oriented strand board (OSB) is fabricated. A rotary drum having closely-spaced cross baffles like those used for the drying of wood chips can be easily used for the drying of strands. If one increases the spacing of the cross baffles, the heat transfer from the drying gas to the bulk material becomes too small. Apart from this, they have the disadvantage of relatively high fabrication and assembly costs.
Baffles for bulk materials are additionally described for a rotary drum in DE-B 18 04 154. These baffles for bulk material have radially extending cell walls which are partly permeable and which subdivide the cross section of the drum into equal sectors which interconnect via openings with one another. The cell walls are formed with ribs on their sides which face rearwardly with respect to the sense of the drum rotation and which extend approximately perpendicularly from the walls and run parallel to the drum axis. In addition, on the inner periphery of each sector, a lifting scoop rib is provided. With this arrangement, the bulk material trickle from the baffles of each sector over a uniform time. At the last part of the drum rotation, however, the sectors are only partly filled. The distribution of the bulk material over the cross section of the drum is nonuniform. Within a sector, at least with drums of large cross section, there may be excessive free fall stretches in which the bulk material can be entrained by the drying gas. If the openings in cell walls are selected so that they are too large, this effect is increased while if the openings are chosen to be too small, there is a danger of plugging up.
From DE-C 33 45 118, a generic device for the drying of sugar with a rotary tube is known. The rotary tube is provided along its inner periphery with a multiplicity of peripheral sets of outer scoops uniformly distributed in a ring shape. All outer scoops of one peripheral set extend in the axial direction over a certain length of the rotary tube. The outer scoops of peripheral sets located one after the other are offset from one another. At the center of the rotary tube a structure is fastened to the rotary tube with an elongated carrier which is formed on its external periphery with internal scoops in approximately a ray shape. These trickle baffles are not suitable for the drying of light wood chips and strands. The fall paths in the interior of the rotary tube are too great. In addition, the surfaces of the baffles in total is too small for an effective heat transfer.